This invention relates to a high tensile strength steel sheet having improved formability including increased ductility and improved hole expandability and which is suitable for use as structural or high strength parts to be shaped by press forming or flange forming in automobiles, industrial machinery and equipment, and the like.
In order to make automobiles, industrial machinery, or other equipment lighter, there have been developed many techniques to increase the strength of steel sheets. However, an increase in strength of a steel sheet is normally accompanied by a decrease in its ductility or formability. Therefore, it is difficult to produce a steel sheet having both good formability and high strength.
Among hot rolled steel sheets, those of dual phase steels described in Japanese Patent Application Kokai (Laid-Open) No. 55-44551 (1980), for example, are known to have high strength and good formability. Dual phase steels have a mixed ferritic and martensitic structure and are characterized by having a low yield ratio and high ductility. However, in the case of 60 kilo-grade high tensile strength steels which have a tensile strength (TS) on the order of 60 kgf/mm.sup.2 or 590 MPa, their elongation (El) is about 30% at highest and their strength-ductility balance (TSxEl) is less than 20,000 (in MPa-%). In the case of 80 kilo-grade high tensile strength steels which have a tensile strength on the order of 80 kgf/mm.sup.2 or 790 MPa, their elongation is about 20% at highest and their strength-ductility balance is less than 18,000 (in MPa-%). A further increase in ductility cannot be achieved with dual phase steels.
It is known that transformation-induced plasticity (abbreviated as TRIP) caused by a retained austenite phase can be utilized to significantly increase the ductility of a high strength steel sheet which may be either a hot or cold rolled steel sheet. TRIP is observed in an Si- and Mn-containing carbon steel sheet having a mixed three-phase structure composed of ferrite, bainite, and retained austenite phases by partial transformation of austenite into bainite during cooling after hot rolling or after heating for annealing. It is the phenomenon that stress-induced transformation of the retained austenite phase occurs during deformation of the steel for forming that causes the steel to exhibit a remarkably high elongation.
A hot rolled steel sheet capable of utilizing the TRIP phenomenon is described in Japanese Patent Application Kokai No. 55-145121 (1980), for example. The steel sheet contains 0.40-0.85% C. (all percents concerning steel chemical compositions being by weight in the present specification), and it is produced by subjecting the hot rolled steel sheet to rapid cooling from a temperature in the austenite region to a temperature in the range of 380.degree. C. to 480.degree. C., at which temperature the steel sheet is then kept for a period sufficient to transform the majority of austenite into bainite, thereby forming the above-described mixed three-phase structure. The resulting hot rolled steel sheet has high strength and good ductility, probably on the order of at least 1100 MPa in TS, at least 22% in El, and a value for TSxEl in excess of 23,500. However, due to the relatively high carbon content in the range of 0.40% to 0.85%, the weldability of the hot rolled steel sheet is too low to be useful in the manufacture of automobiles and structural parts.
A cold rolled, high tensile strength steel sheet capable of utilizing the TRIP phenomenon and having high ductility is described in Japanese Patent Application Kokai No. 61-157625 (1986), for example. It contains 0.4-1.8% Si, 0.2-2.5% Mn, and optionally one or more of P, Ni, Cu, Cr, Ti, Nb, V, and Mo in appropriate amounts. It is produced by subjecting the cold-rolled steel sheet to annealing in such a manner that it is heated at a temperature in the intercritial region followed by cooling, during which the steel sheet is kept for a period of from 30 seconds to 30 minutes at a temperature in the range of 500.degree. C. down to 350.degree. C. to form mixed three phase structure of ferrite, bainite, and retained austenite phases.
Japanese Patent Publication No. 62-35461 (1987) describes a process for producing a high tensile strength steel sheet which has a structure comprising at least 10% by volume of a mixed ferrite and retained austenite phase in a martensitic or bainitic matrix. The process comprises heating a steel sheet containing 0.7-2.0% Si and 0.5-2.0% Mn at a temperature in the intercritical region followed by cooling, during which the steel sheet is kept for 10 to 50 seconds at a temperature in the range of 650.degree. C. to 450.degree. C.
Other disclosures of a hot rolled or cold rolled steel sheet having a structure which contains a retained austenite phase and exhibiting good ductility include U.S. Pat. Nos. 5,017,248 and 5,030,208, and Japanese Patent Applications Kokai Nos. 63-4017 (1988), 64-79322 (1989), 1-159317 (1989), 4-28820 (1992), 4-333524 (1992), 4-371528 (1992), and 5-59492 (1993).
However, these high ductility, high tensile strength steel sheets capable of utilizing the TRIP phenomenon of retained austenite, whether hot or cold rolled, have the common drawback that despite their good ductility or high elongation in a tensile test, their press formability is not always improved to a degree predictable from the ductility level so that they cannot be successfully used in fabrication by press forming. It is believed that such deterioration in press formability is attributable to the fact that the local ductility in the press-formed area is greatly deteriorated at a late stage of deformation in press forming, since most of the retained austenite phase has already been transformed into high-carbon martensite by stress-induced transformation before that time. This is particularly significant in flange forming, including hole expansion. As a result, the hole expandability of these steel sheets is inferior to that of conventional high tensile strength, cold rolled steel sheets of the low carbon type. This is considered to be caused by a high-carbon martensite phase formed by stress-induced transformation during punching for forming an initial hole to be expanded. The high hardness of the martensite phase causes the formation of minute cracks around the initial hole, which are extended or propagated in the subsequent hole expansion stage, thereby deteriorating the hole expandability.
In conventional processes for producing steel sheets having the above-described mixed three-phase structure, a change of strength level of a steel sheet is inevitably accompanied by a change in carbon content. However, a decrease in carbon content leads to a decrease in volume fraction of retained austenite in the steel, which makes it difficult to improve the ductility of the steel sufficiently by the TRIP phenomenon.
Japanese Patent Application Kokai No. 4-341523 (1992) discloses two processes for producing a hot rolled steel sheet having a structure comprising a retained austenite phase and containing 0.10-0.35% C, 1.0-3.0% Si, 0.5-2.5% Mn, and one or more of Cr, Al, P, and Ni. In a first process, after hot rolling is performed with a finish rolling end temperature below 950.degree. C., the hot rolled steel sheet is cooled to a temperature between 600.degree. C. and 800.degree. C. at a rate of 1.degree.-200.degree. C./sec, then slowly cooled to a temperature immediately above the pearlite transformation temperature at a rate of 30.degree. C./sec or lower, and further cooled to a coiling temperature between 300.degree. C. and 500.degree. C. at such a rate that pearlite transformation can be inhibited. In a second process, hot rolling is performed at a high reduction rate of at least 80% with a finish rolling end temperature below 850.degree. C. The hot rolled steel sheet is then directly cooled to a coiling temperature between 300.degree. C. and 500.degree. C. at such a rate that pearlite transformation can be inhibited. Both processes provide a steel sheet having high strength and good ductility and press formability including good hole expandability. However, the hot rolled steel sheet is disadvantageous in that addition of a relatively large amount of Si is mandatory, which causes a eutectic reaction between SiO.sub.2 and FeO significantly during heating in the hot rolling step, resulting in the uneven formation of low melting, high-Si scales on the steel surface. As a result, the resulting hot rolled steel sheet has an uneven surface after pickling for descaling, thereby impairing the surface quality significantly.